1. Field of the Invention
The present invention relates to a photovoltaic element with excellent characteristics and high reliability and a method for producing the photovoltaic element, and more specifically to a photovoltaic element with excellent characteristics and a reduced loss in energy conversion efficiency due to the formation of a bypass diode under a current collecting electrode and a method for producing the photovoltaic element.
2. Related Background Art
A thin film type solar cell employing amorphous semiconductor is considered promising due to its advantages such as its capabilities of forming a large-area solar cell, making the film thickness of a semiconductor thinner and depositing a film on an arbitrary substrate, as compared to a single-crystal or polycrystal type solar cell.
An amorphous silicon type solar cell is formed, for example, by stacking p-, i- and n-type thin amorphous silicon layers on a substrate. Also, for improving the energy conversion efficiency, there is contemplated a so-called double or triple cell structure in which two or more of the above-mentioned pin junctions are superposed in series. At the light incident side and the back side of the above-mentioned semiconductor, there are formed a pair of electrodes, namely an upper electrode and a lower electrode. In the amorphous silicon type solar cell, because of the generally high sheet resistance of the semiconductor itself, there is required a transparent upper electrode covering the entire area of the semiconductor, which is usually composed of a transparent conductive film such as of SnO.sub.2 or ITO. Such transparent conductive film functions also as an anti-reflective film. On the upper electrode mentioned above, there is provided a current collecting grid electrode which is formed into a comb-shaped pattern so as not to hinder the entry of light, in other words, the irradiation of light, and a busbar is provided in order to collect the current from the grid electrode.
As an electric power supply source, a single solar cell (photovoltaic element) is incapable of supplying a sufficient output voltage. For this reason, it is necessary to use plural solar cells in parallel or serial connection. The largest difficulty in utilizing the plural cells (elements) in serial connection as described above relates to the situation wherein no electric power is generated due to a part of the cells being shadowed from the sunlight, for example, by a building or by accumulated snow. A serially connected solar cell module can no longer generate an electric power even though other cells in the module still generate electric power, and the total voltage generated by the normally functioning cells is applied, as a backward voltage, to such a shadowed cell. When such a backward voltage exceeds the tolerable voltage of the element, destruction thereof may result. In order to prevent such a problem in the electric power generation or the destruction of the element, it is necessary to connect, for each of the serially connected elements, a diode parallel to the element but in a direction opposite to that of the semiconductor junction of the element. Such a diode is generally called a bypass diode.
The application of the bypass diode to the solar cell is, for example, disclosed in Japanese Patent Application Laid-Open No. 5-152596, in which a mold-packaged diode is connected in parallel to each solar cell. FIG. 9 is a schematic view showing an example of the solar cell module utilizing such bypass diode. FIG. 9 shows a solar cell module 91 connected to bypass diodes, solar cells 92, bypass diodes 93, wirings 94, wirings 95 for serially connecting the solar cells 92, a glass plate 96, an encapsulating resin 97, and a back plastic material 98. As the diode 93 has a thickness of about 3 mm in diameter in the case of a usual axial diode, the encapsulating resin 97 has to be made correspondingly thick.
There is proposed a method of incorporating a diode in a semiconductor constituting a solar cell, as another method of the prior art, because the attachment of an independent diode to the solar cell is considered to increase the thickness of a solar cell module by the thickness of the diode and complicate the manufacturing process with respect to the wiring work. Such a proposal is disclosed, for example, in Japanese Patent Application Laid-Open No. 4-42974, in which a pn junction functioning as a solar cell and a pn junction serving as a bypass diode are formed on the same substrate in such a manner that they are mutually connected in parallel.
However, with respect to such a conventional photovoltaic element with the bypass diode, (1) in the photovoltaic element employing an amorphous semiconductor film formed on the aforementioned substrate, there has not been disclosed a method of forming the bypass diode on the same substrate, (2) the configuration and the producing method require a masking process, which is complex and lacks freedom in size, and (3) the area of the bypass diode constitutes a loss in the effective area of the photovoltaic element; in other words, it is necessary to increase the area of the solar cell by the area of the bypass diode.